Field of the Invention
The invention relates to an imaging system based on nuclear magnetic resonance, including an RF coil device for the excitation of nuclei in a patient region to be examined and for the detection of signals emitted by the excited nuclei, the RF coil device having at least one first coil element, and at least one electrically completely insulated second coil element magnetically coupled to the first coil element for amplifying the signals emitted from the region to be examined. A corresponding imaging system is disclosed, for example, by German Patent DE 35 00 456 C2.
During operative interventions in the human organism, the requirement that an imaging system be used to monitor the operative procedures as well as the region of the body undergoing treatment is being imposed more and more frequently. In principle, all of the imaging methods which are customary at the present time in medical technology are suitable for that purpose, such as computer tomography (CT), ultrasound or magnetic resonance (MR) methods. On one hand, CT methods make use of ionizing X-rays, which are known to put a strain both on the patient's body and on the surgeon's body. On the other hand, during ultrasound monitoring of tumor operations, the image quality can be impaired by artifacts as a consequence of blood accumulations. As a result, the important statement of diagnosis regarding whether it has been possible to completely eliminate a tumor or whether residues still remain, becomes impossible under certain circumstances or is at the very least rendered more difficult. By contrast, MR methods exhibit a better image quality precisely during the imaging of soft tissue, so that they are particularly well suited to the application described.
MR imaging systems, which generate sectional images of an object to be examined, in particular of a human body or body part, using nuclear magnetic resonances, are known per se. In that case, the body to be examined is introduced into a strong, homogeneous, static magnetic field, a so-called background field, which effects alignment of nuclear spins of atomic nuclei, in particular of hydrogen atom nuclei (protons) bound to water, in the body. Those nuclei are then excited to effect precessional motion through the use of radio frequency excitation pulses. After the end of a corresponding radio frequency excitation pulse, the atomic nuclei precess with a frequency depending on the strength of the background field and then settle back into a preferred direction, predetermined by the background field, due to their spins at a predetermined relaxation time. Computational and/or measurement analysis of the integral, radio frequency signals from the nuclei can be used to generate an image, with regard to a layer of the body, from the spatial spin density or from the distribution of the relaxation times. Linear field gradients are used to assign the nuclear resonance signal, which can be detected due to the precessional motion, to the location where it originated. For that purpose, corresponding gradient fields are superposed on the background field and controlled in such a way that the nuclei are excited only in a layer to be imaged. An RF coil device is necessary both for radio frequency (RF) excitation of the nuclear spins and for detection of the response signals from the nuclei. Imaging systems based on those physical effects are also known under the designations nuclear spin tomography, nuclear magnetic resonance (NMR) tomography or magnetic resonance imaging (MRI).
A paper entitled "Intraoperative Diagnostic and Interventional Magnetic Resonance Imaging in Neurosurgery" by V. M. Tronnier et al., in Neurosurgery, Vol. 40, No. 5, May 1997, pages 891 to 902 describes an interventional MR imaging system constructed for use in neurosurgery for the purpose of monitoring operative interventions. In that case, use is made of an RF coil formed of two separate partial areas connected through the use of contact connectors. The first separate coil part remains unsterile, whereas the second coil part and the two contact connectors must be sterile since they are in direct proximity to the treatment region and, consequently, must satisfy the specific requirements of sterility during operations. In that context, sterility is to be understood to mean the maximum number of germs in the surroundings of the treated body parts, in particular of open operation wounds, which is usually permissible in the case of medical interventions. In order to avoid consequential infections of the wounds, absolute freedom from germs is striven for in that case. The structure described consequently requires special measures for sterilization which can turn out to be complicated precisely for the contact connectors. The same applies to electrical supply leads of RF coils if they are likewise routed through the sterile area, as is customary in other devices.
German Patent DE 35 00 456 C2 discloses an RF coil device for an MR imaging system in which, in addition to a first RF coil connected to the electronic unit, a second RF coil is also used. The second RF coil is electrically completely insulated from the surroundings. The two coils are magnetically coupled, with the result that signals received by the second RF coil from a region to be examined are coupled over into the first RF coil, from where they reach the connected electronic evaluation unit. In that case, the second RF coil is closer to the region to be examined than the first RF coil. Signal amplification is the aim of the specific configuration disclosed, having two magnetically coupled RF coils. Since the magnetic flux is concentrated through the second coil in a targeted manner in the region to be examined, the desired increase in the signal levels is thus achieved. However, there are no statements in that publication regarding the specific requirements which have to be satisfied by the RF coil device if it is to be used in connection with operative measures on the region to be examined. Rather, the RF coil devices disclosed in the exemplary embodiments of that device would make an operative intervention taking place in parallel with the MR imaging appear to be impossible.